linux-kernel - Re: [PATCH v2 11/14] Documentation: dt: binding: omap: am43x timer

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Date:	Wed, 29 May 2013 10:39:18 +0200
From:	Benoit Cousson <b-cousson@...com>
To:	"Mohammed, Afzal" <afzal@...com>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Jon Hunter <jgchunter@...il.com>,
	Russell King <linux@....linux.org.uk>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Grant Likely <grant.likely@...aro.org>,
	Benoit Cousson <benoit.cousson@...aro.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 11/14] Documentation: dt: binding: omap: am43x timer

Hi Afzal,

On 05/29/2013 10:06 AM, Mohammed, Afzal wrote:
> Hi Jon,
> 
> On Wed, May 29, 2013 at 03:35:10, Stephen Warren wrote:
>> On 05/28/2013 03:25 PM, Jon Hunter wrote:
> 
>>>>  			ti,am335x-timer	(applicable to AM335x devices)
>>>>  			ti,am335x-timer-1ms (applicable to AM335x devices)
>>>> +			"ti,am4372-timer-1ms", "ti,am335x-timer-1ms" for AM43x 1ms timer
>>>> +			"ti,am4372-timer", "ti,am335x-timer" for AM43x timers other than 1ms one
> 
>>> If you are adding more compatibility strings, then this implies that the
>>> AM43x timers are not 100% compatible with any other device listed (such
>>> as am335x or any omap device). That's fine but you should state that in
>>> the changelog. If the AM43x timer registers are 100% compatible with
>>> existing devices you should not add these.
>>
>> I'm not sure that's true; .dts files should always include a compatible
>> value that describes the most specific model of the HW, plus any
>> baseline compatible value that the HW is compatible with. This allows
>> any required quirks/fixes/... to be applied for the specific HW model
>> later even if nobody knows right now they'll be needed. Hence, defining
>> new compatible values doesn't necessarily mean incompatible HW.
> 
> Stephen took words out of my finger ;)
>  
> Some explanations,I don;t 
> 
> 1. first compatible should be exact device [A], followed by compatible
> model (if one)
> 2. Minor effort in getting DT right the first time may help prevent
> difficult effort later modifying it (if a necessity comes), considering
> the fact that DT sources has  to move out of Kernel at some point of
> time. And DT is not supposed to be modified, which may cause difficulty
> for the users (I had been a minor victim of this during rebase).
> 
> As we both were in GPMC land earlier, an example,
> 
> If my memory is right, GPMC IP in am335x is rev 6, and IP has 8 chip
> select, but one is not pinned out. Now assume that same IP is integrated
> in another SoC (probably OMAP4 has rev 6). Here if we use same compatible
> for both, driver cannot handle it properly (w/o knowledge about platform).
> But if exact compatible is mentioned, without modifying DT (which should
> be considered as a firmware) just by modifying Kernel, deciding based on
> compatible would help achieve what is required.

That's true for the DTS itself, but here your are changing the binding
documentation which is supposed to reflect the driver "interface" in the
Device Tree model description.

Since the driver does not support any new compatible string, you should
not update the binding.

The driver and the binding will have to be changed the day you will have
to update the driver to handle a bug / feature specific to that revision
(ti,am4372-timer).

Since this series does not seems to update the driver, you should not
update the bindings.

Regards,
Benoit


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